THE RELATIONSHIP OF ENDOPHYTIC FUNGI TO THE GAMETOPHYTE OF THE FERN SCHIZAEA-PUSILLA

Schizaea pusilla is a rare and threatened fern restricted in North Ame rica to acidic bogs of Nova Scotia, Newfoundland, and New Jersey. The gametophyte lives in close association with two endophytic fungi. To c haracterize the nature of this fern's relationship with these fungi, w e introduced axenic gametophytes to bog soil for colonization. Followi ng colonization, the endophytic fungi were isolated and reintroduced t o axenic gametophytes. The gametophytes introduced to bog soil were co lonized by an aseptate fungus that formed vesicles and arbuscules with in the gametophyte. However, culture of colonized gametophytes produce d two fungal isolates: an aseptate fungus (fungus B) and a septate fun gus (fungus A). Upon reintroduction of fungal isolates to axenically g rown gametophytes, the aseptate fungus demonstrated a positive growth response to the presence of the gametophytes and colonized the gametop hytes without harm to the host. The septate fungus did not exhibit any specific recognition but contacted the gametophytes randomly, leaving a large percentage of the host nonviable. We propose that the relatio nship of the septate fungus to the gametophyte of S. pusilla is nonmyc orrhizal while the relationship of the aseptate fungus to the gametoph yte is mycorrhizal. Furthermore, based on lack of nutrient availabilit y in local soils, formation of specialized structures in the gametophy te for harboring fungi, and dependence of the fern on fungal presence for completion of its life cycle, we propose that S. pusilla maintains an obligatory relationship with the aseptate mycorrhizal fungus.